This invention relates to multimode waveguides and methods of fabricating them.
An optical pulse launched into a multimode optical waveguide excites many modes, each traveling at a different group velocity. Such waveguides suffer from multimode dispersion. At the far end of the waveguide, the pulse is spread out in time by an amount that is proportional to the length of the waveguide because of the different group velocities of the modes. Such multimode dispersion can severely limit the information-carrying capacity of the waveguide.
It is known that multimode dispersion in optical waveguides can be reduced by deliberately enhancing coupling among the various modes in the waveguide. In accordance with the teachings of U.S. Pat. Nos. 3,666,348 Marcatili, 3,687,514 Miller and 3,912,478 Presby, mode coupling can be produced by causing variations in such waveguide parameters as core radius, core refractive index and waveguide axis. Since the optical power carried in the waveguide transfers back and forth between slow and fast modes, averaging takes place, so that the propagating light pulse travels at an average group delay, with a resultant reduction in pulse broadening. In the presence of such mode coupling the pulse width increases only as the square root of the waveguide length.
Coupling optical power among the guided modes of a multimode optical waveguide can significantly reduce the dispersion caused by intermodal delay differences. Perturbations cause coupling among the guided modes and will, in general, also cause coupling from the guided to the unguided modes. The latter type of power transfer is undesirable since it reduces the strength of the transmitted signal.
Coupling induced losses are determined by the strength of the coupling among the higher order modes, whereas the reduction in pulse dispersion is determined by the average strength of the coupling among the guided modes. For a given loss penalty the benefit in decreased pulse broadening improved if the average coupling strength can be made higher relative to the coupling strength which governs the losses. For example in the aforementioned U.S. Pat. No. 3,666,348 Marcatili the coupling is restricted to selected pairs of guided modes by controlling the spatial periodicity of the coupling mechanism. In particular, the spatial periodicity is made equal to the beat wavelength for the two modes.
The perturbations which cause mode coupling take many forms. For example, small bubbles at the core-cladding interface, bends in the waveguide, and variations in the diameter of the guide are all perturbations which have been shown to cause mode coupling. Mode coupling theory shows that the coupling between two modes is proportional to the power spectrum of the perturbation. Perturbations of random lengths promote coupling between all modes, guided and unguided. FIGS. 2 and 3 of the Miller et al patent show methods of making waveguides with random perturbations in response to a source of noise. However, it is desired to produce perturbations having a power spectrum which is high for the guided modes. In order to do this, the Miller et al patent shows, in FIG. 5, a technique of band pass filtering the noise source before using the noise to cause perturbations in the waveguide fabrication process. The theory is that such a process will cause perturbations having a spatial periodicity equal to the beat wavelength between the modes to be coupled. Theoretically, such a technique would produce perturbations promoting coupling only between the guided modes with no power coupling to the unguided modes. However, in practice, the fabrication technique shown in FIG. 5 of the Miller et al patent is difficult to carry out.